Horrible plugin that shows up on my contact form 7 as a large text field followed, on the next line, by "-9=4". That's hard to interpret, even for me. Then if you enter the incorrect answer, you either get a timeout or an error even if you subsequently enter a correct answer. Notice how the developer does not show you any screenshots of the plugin working? That's because it's awful.

After installing I tested adding of a comment as registered user - I had problem with this - no capcha asked, bud comment did not save and throwed a capcha error.
Unchecking "comment form " in "Math Captcha settings" solved the problem.

I started getting far too many spam comments. The price you pay as a site's popularity grows. I wanted something to slow the spammers but did not want a plugin that bloats my site and slows it down. Tested this simple plugin and...
Spam comments came almost to a complete halt. My pages load just as fast but I have a cdn and cache plugin. I did not want a heavy plugin that was contacting outside databases and slowing down my site and possibly negatively effecting serp results because of longer load times. This plugin works. It's effective. It does not slow my site at all according to the waterfall at gtmetrix.com. People leave just as many real comments as before.